1. Field of the Invention
The present invention relates to an image inspection system, for example, a system for inspecting images of a subject having a layered structure, such as a semiconductor wafer or a liquid crystal substrate.
Priority is claimed on Japanese Patent Application No. 2004-374522, filed Dec. 24, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, image inspection systems have been widely known, which are used for obtaining images of subjects to be inspected, and inspecting patterns formed on the subjects or inspecting for the presence/absence of defects or dust on surfaces of the subjects.
For example, an image inspection system used on subjects having layered structures, such as semiconductor wafers or liquid crystal substrates, is known. Such an image inspection system has an observation optical system including a microscope for obtaining an enlarged image, and an autofocusing mechanism for performing focusing of the subject by using a branch optical path separated from the observation optical system. In this system, a stage on which a subject is disposed is driven along the optical axis, so as to perform autofocusing.
For example, Reference 1 (Japanese Unexamined Patent Application, First Publication No. H11-95091, specifically, pp. 4-6, and FIGS. 1 and 2) discloses an autofocusing microscope having objective lenses which can be switchably used. The autofocusing microscope employs a knife-edge focusing method for detecting a focal point by irradiating a subject with laser light. The autofocusing microscope has an autofocusing optical system branching away from an observation optical system, in which a series of color compensating lenses for correcting chromatic aberration of each objective lens is movably provided. An amount of correction assigned to each objective lens is stored in a ROM. When the objective lens is switched, the focal point is also switched with reference to the ROM.
Reference 1 also discloses that when the operator would like to observe a specific part of a subject which has structural steps, the autofocusing mechanism is used for focusing on the specific part by setting an offset value.